This invention relates to a telephone line interface circuit provided in telephone equipment.
FIG. 1 is the block diagram illustrating the configuration of the conventional telephone line interface circuit. In FIG. 1 an arrester circuit 4 is connected between telephone lines L.sub.1 and L.sub.2 to prevent the telephone equipment from the surge by thunder. A filter 1 is inserted into the telephone line L.sub.1 and passes only the call signal of 16 Hz (75 V) among the signals sent in through the telephone lines L.sub.1 and L.sub.2. The changeover circuit 2 is driven when the telephone equipment is switched over from the call waiting state to the conversation state or from the conversation state to the call waiting state, has a fixed contact 2a connected to the input of the filter 1, the fixed contact 2b connected to the input of a call income detecting circuit 3 and a movable contact 2c connected to the output of the filter 1, and is controlled by a hook switching control signal A output from the controller 19.
During the call waiting state, that is, when the handset is on-hook, a contact 2b, as illustrated, is closed to permit the call signal after passing through the filter 1 to be entered into the call income detecting circuit, and during the conversation state, that is, when the handset is off-hook, the contact 2a is closed to send speech signals from the telephone lines L.sub.1 and L.sub.2 into the internal speech lines L.sub.3, L.sub.4 directly. The call incoming detecting circuit 3 detects the call signal sent from the telephone lines L.sub.1 and L.sub.2 and sends out the call incoming detecting signal B.
The dial pulse sending circuit 8, which sends dial pulse signals to the telephone lines L.sub.1 and L.sub.2, is located on the more inside of the equipment than the connecting point of the telephone line L.sub.2 with the call income detecting circuit 3. The dial pulse sending circuit 8 is controlled by the dial sending control signal c sent out from the controller 19 and sends out the specified dial pulse signals by ON/OFF operation. A protection circuit 18 protects various circuit elements, for example, photocouplers 6 and 7, a diode bridge circuit 5 and a constant current circuit 11, located on the more inside of equipment than above mentioned every part from the surge of high voltage, etc.
The photocouplers 6 and 7 are provided to detect the inversion of the polarity of the DC voltage between the lines L.sub.1 and L.sub.2. The photocoupler 6 operates to make the light receiving device 6b conductive when the line L.sub.2 side is positive and the line L.sub.1 side is negative. The photocoupler 7 operates to make a light receiving device 7b conductive when the line L.sub.1 side is positive and the line L.sub.2 side is negative. A current control resistor 20 connected to light emitting devices 6a and 7a has high resistance (e.g., 10 k.OMEGA.) so that the impedance of the telephone equipment in the conversation state is not affected by the resistor 20.
The diode bridge circuit 5 makes the voltage polarities between the lines L.sub.1 and L.sub.2 uniform in one direction to output them to internal speech lines L.sub.3 and L.sub.4. By the diode bridge circuit 5, the internal speech line L.sub.3 is always of positive polarity and the internal speech line L.sub.4 is always of negative polarity.
The constant current circuit 11 makes the impedance of the telephone equipment from the lines L.sub.1 and L.sub.2 to adapt to the specified standard. A capacitor 12 removes the direct current component included in the output of the diode bridge circuit 5. A transformer 13 sends out the speech signals from the lines L.sub.3 and L.sub.4 through the secondary side, and sends out the speech signals from the secondary side to the internal speech lines L.sub.3 and L.sub.4.
The controller 19 has a NAND element 19a connected with the output of the light receiving devices 6a and 7b, the loop current detector 19b connected with the output of the NAND element 19a and the polarity inversion detector 19c connected with the output of the light receiving devices 6b and 7b. In addition, it has the hook changeover control 19d to switch over the changeover circuit 2 in response to the condition of the handset, the call income detector 19e recognizing the call incoming by receiving the output B of the call incoming signal detector circuit 3 and the dial pulse sending control 19f to control the dial pulse sending circuit 8. The loop current detector 19b recognizes, by the signal of the logic level "H" coming from the NAND element 19a, that the current flows through the direct-current loop consisting of the telephone lines L.sub.1 and L.sub.2, the diode bridge circuit 5, the internal speech lines L.sub.3 and L.sub.4 and a constant-current circuit 11, i.e., that the telephone equipment is in the conversation state. The polarity inversion detector 19c grasps which of the light receiving devices 6b and 7b is in the conductive state and recognizes the inversion of the voltage polarity between the lines L.sub.1 and L.sub.2. Since this polarity inversion is carried out by the telephone office at the time of charging and the end of telephone conversation, the time registering is performed on the basis of the detected information of this polarity inversion.
In the configuration as described above, when the changeover circuit 2 closes the contact 2b as illustrated and the telephone equipment is in the call waiting state, if the call signal comes from the lines L.sub.1 and L.sub.2, this call signal is entered to the call incoming detecting circuit 3 and the call incoming detecting signal B is sent out. Receiving this signal B, the controller 19 operates to ring a call bell.
When the handset is hooked off, the changeover circuit 2 closes the contact 2a, resulting in the conversation state of the telephone equipment. In the conversation state, if the line L.sub.1 side is of positive polarity, a loop current flows from the circuit L.sub.1 through the diode bridge circuit 5, the internal speech line L.sub.3, the constant current circuit 11, the internal speech line L.sub.4 and the diode bridge circuit 5 to the line L.sub.2. At this time, a part of the loop current flows into the light emitting device 7a of the photocoupler 7, thus the light receiving device 7b becomes conductive, then the loop current detector 19b recognizes that the loop current flows and that the polarity inversion detector 19c recognizes that the line L.sub.1 side is of positive polarity. If the line L.sub.2 side is of positive polarity, the loop current and the positive polarity of the circuit L.sub.2 side are recognized by the conductivity of the light receiving element 6b of the photocoupler 6.
In the conversation state, the voltage polarity between the lines L.sub.1 and L.sub.2 is inverted at every time of charging and the end of the telephone conversation. Since the direction of the loop current is changed by the polarity inversion, the conductive state of the light receiving device 6b and 7b is changed over and the polarity inversion is recognized.
In the conventional telephone line interface circuit as described above, the dial pulse sending circuit 8 is located nearer to the outside of the equipment, that is, nearer to the lines L.sub.1 and L.sub.2, than the photocouplers 6 and 7. The reason for this is described below. That is, assuming that the dial pulse sending circuit 8 is located nearer to the inside of equipment than the photocouplers 6 and 7, the impedance of the equipment from the lines L.sub.1 and L.sub.2, when the dial pulse sending circuit 8 is OFF (dial break), is determined by the resistance of the current control resistor 20. If the resistance of the resistor 20 is about 10 k.OMEGA. as described above, the impedance of the telephone equipment at the dial break is too low for the telephone office to detect the dial break, and the telephone office cannot detect the dial pulse signal sent from the dial pulse sending circuit 8. Then, to make the impedance of the telephone equipment from the lines L.sub.1 and L.sub.2 at the dial break at least 100 k.OMEGA., if a resistor having 100 k.OMEGA. or larger resistance is used as the resistor 20, the current sufficient to operate the photocouplers 6 and 7 cannot be supplied. Therefore, in order to solve simultaneously the above-mentioned two troubles, it is necessary to locate the dial pulse sending circuit 8 nearer to the outside of equipment outward than the photocouplers 6 and 7.
However, since the dial pulse sending circuit 8 is located nearer to the outside of equipment than the photocouplers 6 and 7, it is necessary that the dial pulse sending circuit 8 has a construction permitting a current to pass in both directions to make it possible to send out the dial pulse both for the case where the circuit L.sub.1 is of positive polarity and for the case where the circuit L.sub.2 is of positive polarity. Furthermore, the capability of sending out the dial pulse of high accuracy is required for the dial pulse sending circuit 8. Therefore, to meet this requirements, it is necessary to use a special relay for the dial pulse sending circuit 8, thereby resulting in such problems as increase in cost and in scale of the circuit.
The object of this invention is to provide a telephone line interface circuit which uses a low-cost and small-scale circuit as a dial pulse sending circuit.